1. Technical Field
The invention relates to a high stability electron beam generator for processing material, comprising an electrically heated cathode, a perforated anode, and a control electrode lying at a potential which is more positive than the cathode and disposed between the cathode and the anode.
2. Underlying Prior Art
In the known electron beam generating system for processing material, the electron beam is generated and controlled by three electrodes, cathode, perforated anode and Wehnelt cylinder. Directly or indirectly heated H-needle, ribbon- or bolt-cathodes consisting of tungsten are employed as the cathode and cathodes consisting of lanthanhexaborate are also employed.
The electrons of the electron beam are emitted from the cathode due to the heating and are subsequently accelerated. For example, the cathode can be charged with a high negative voltage, whereas the anode lies at grounded potential. The electrodes are accelerated toward the anode in the electrical field generated in that manner, pass through the anode perforation and arrive in the field-free space.
In the previous systems, the third electrode, the Wehnelt or lattice electrode, serves for controlling the current of the beam. The function of the Wehnelt electrode is described, for example, in the publication M. Blocke, Zeitschrift fur angewandte Physik, Vol. 3, 1951, pp. 441 through 449, "Elementare Theorie der Elektronenstrahlerzeugung mit Trioden-systemen".
When the negative voltage of the Wehnelt electrode is increased beyond a specific value, then further electrons are incapable of leaving the cathode surface, i.e., there are no electrical field vectors which accelerate the electrons to the anode directed to the anode at the cathode surface. The emission is blocked. When the Wehnelt voltage is reduced, then the anode penetration coefficient increases. Given a decreasing Wehnelt voltage, larger and larger areas of the cathode are released for emission, whereby the emission current increases. Thus, the variable negative voltage of the Wehnelt cylinder in comparison to the cathode allows a control of the beam current.
A characteristic of these systems is described in greater detail below. It derives from the structural disposition of the Wehnelt cylinder and due to the prescribed Wehnelt voltage which is required for setting a desired beam current, a fixed beam geometry which can only be changed by means of changing the structural shape or the Wehnelt voltage. However, a change of the Wehnelt voltage again leads to a change of the beam current. This rigid dependency of the beam current upon the Wehnelt voltage and the beam shape fixed by the Wehnelt arrangement and Wehnelt voltage are disadvantageous.
Added thereto as a further disadvantage is that, in the standard operating mode, the cathode is operated in the space charge range, whereby the beam value lies below the value which would theoretically derive given operation of the cathode in the saturation range. Resulting therefrom is that the efficiency of the electron beam generator does not assume the optimally attainable value.